1. Technical Field
The present disclosure relates to a microelectromechanical gyroscope with improved start-up phase, to a system including the microelectromechanical gyroscope, and to a method for speeding-up the start up phase.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has become increasingly widespread in various sectors of technology and has yielded encouraging results especially in the production of inertial sensors, microintegrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS of this type are usually based upon microelectromechanical structures comprising at least one mobile mass connected to a fixed body (stator) by means of springs and mobile with respect to the stator according to pre-set degrees of freedom. The mobile mass is moreover coupled to the fixed body via capacitive structures (capacitors). The movement of the mobile mass with respect to the fixed body, for example on account of an external stress, modifies the capacitance of the capacitors, whence it is possible to trace back to the relative displacement of the mobile mass with respect to the fixed body and hence to the force applied. Conversely, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the mobile mass to set it in motion. Furthermore, to obtain electromechanical oscillators the frequency response of the inertial MEMS structures is exploited, which is typically of a second-order lowpass type.
Many MEMS systems (in particular, all electromechanical oscillators and gyroscopes) use driving devices that have the task of keeping the mobile mass in oscillation.
The gyroscopes have a complex electromechanical structure, which comprises two masses that are mobile with respect to the stator and coupled together so as to have a relative degree of freedom. The two mobile masses are both capacitively coupled to the stator. One of the mobile masses is dedicated to driving (driving mass) and is kept in oscillation at the resonance frequency. The other mobile mass (sensing mass) is dragged along in oscillatory motion and, in the event of rotation of the microstructure with respect to a pre-set axis with an angular velocity, is subject to a Coriolis force proportional to the angular velocity itself. In practice, the sensing mass operates as an accelerometer that enables detection of the Coriolis force.
To enable actuation and produce an electromechanical oscillator where the sensor performs the role of selective frequency amplifier, with second-order transfer function of a lowpass type and high merit factor, the driving mass is equipped with two types of differential capacitive structures: driving electrodes and driving sensing electrodes. The driving electrodes have the purpose of sustaining the self-oscillation of the mobile mass in the direction of actuation. The driving sensing electrodes have the purpose of measuring, through the transduced charge, the position of translation or rotation of the sensing mass in the direction of actuation.
The U.S. Pat. No. 7,305,880 describes a system for controlling the rate of oscillation of the gyroscope, comprising a reading system including a differential read amplifier, a highpass amplifier, and an driving-and-control stage, operating in a time-continuous way. All the components that form the reading system are of a discrete-time analog type and, in particular, are provided by means of fully differential switched-capacitor circuits.
The U.S. Pat. No. 7,827,864 describes an improvement of the previous control system, where the control loop comprises a filter having the purpose of reducing the offset and the effects of components and any parasitic coupling, operating on the overall gain and phase of the feedback loop.
To obtain a self-sustained oscillation of an electromechanical oscillator, at a constant amplitude, the loop according to the known art complies with the Barkhausen stability criterion, which is a mathematical condition to determine when a loop comprising linear blocks oscillates, and it is given by the following equations (1a) and (1b):
                    {                                                                                                                                          G                      LOOP                                        ⁡                                          (                                              ω                        =                                                  ω                          dr                                                                    )                                                                                        =                1                                                                                                          φ                  ⁡                                      (                                                                  G                        LOOP                                            ⁡                                              (                                                  ω                          =                                                      ω                            dr                                                                          )                                                              )                                                  =                                                      k                    ·                    2                                    ⁢                  π                                                                                                                            (                              1                ⁢                a                            )                                                                          (                              1                ⁢                b                            )                                          
where the first equation means that the gain GLOOP of the loop at the driving frequency ωdr must be unitary, and the second equation means that the phase shift φ (accumulated by the signal in a round trip of the loop) must be an integer multiple of 2π.
During the start-up phase of the electromechanical oscillator, equations (1a) and (1b) must be varied in such a way to obtain a time evolution of the oscillation up to reaching a desired oscillation amplitude, i.e., the gain GLOOP must be higher than a unitary gain. It follows that the following equations (2a) and (2b) are to be complied with during the start-up:
                    {                                                                                                                                          G                      LOOP                                        ⁡                                          (                                              ω                        =                                                  ω                          dr                                                                    )                                                                                        >                1                                                                                                          φ                  ⁡                                      (                                                                  G                        LOOP                                            ⁡                                              (                                                  ω                          =                                                      ω                            dr                                                                          )                                                              )                                                  =                                                      k                    ·                    2                                    ⁢                  π                                                                                                                            (                              2                ⁢                a                            )                                                                          (                              2                ⁢                b                            )                                          
Under the condition sets out by equations (2a) and (2b) the oscillations of the electromechanical oscillator are free to evolve from the noise (always present in real systems). The aforementioned evolution follows, substantially, the following steps:                i. At a starting point, when the system is turned on, an error signal (the error signal being the difference between the target amplitude of oscillation and the effective amplitude) is considerably high; this error causes the control voltage of the VGA to its maximum value in a time which depends on the control strategy adopted.        ii. The gain of the loop is at the maximum value, thus the condition sets out by (2a) is assured and the system evolves from the noise signal;        iii. after a certain start-up transitory period, during which the oscillation amplitude rises, a desired oscillation amplitude is reached; at this point the condition sets out by (1a) must be respected.        
The term transitory period at step (iii), indicates the period during which the oscillating mass of the electromechanical oscillator passes from a stationary condition (the mass does not oscillate) to an oscillating state where the mass oscillates at a controlled, desired, amplitude, or around the desired amplitude.
In the above identified systems, according to known embodiments, the start-up phase evolves slowly, since it evolves just from the noise.